The present invention relates to an ultrasonic imaging apparatus and, more particularly, to an ultrasonic imaging apparatus for a mechanical scanning system using a plurality of ultrasonic transducer elements.
An ultrasonic imaging apparatus is used in ultrasonic diagnosis equipment. The ultrasonic imaging apparatus can employ a sector scanning system for sector-scanning an object to be examined and a radial scanning system for 360.degree.-scanning an object to be examined. The ultrasonic imaging apparatus of the radial scanning system is used together with an endoscope, so that an object to be examined in a body cavity is scanned through 360.degree. about an ultrasonic transducer, thus providing a tomographic image.
In an endoscopic ultrasonic diagnosis apparatus, an ultrasonic transducer is rotatably arranged at the distal end portion of an endoscope, the distal end portion of the endoscope is inserted in a body cavity, and the ultrasonic transducer is rotated. Ultrasonic wave transmission/reception operations are performed 512 times per rotation of the ultrasonic transducer to radially scan the object. An image signal obtained by the radial scan is displayed as a tomographic image.
In the ultrasonic diagnosis apparatus of the radial scanning type, an ultrasonic diagnosis apparatus which can arbitrarily switch a focal length of an ultrasonic wave has been developed. With this ultrasonic diagnosis apparatus, a rotary ultrasonic transducer comprises a plurality of ultrasonic transducer elements which are divisionally arranged in the circumferential direction, and have different ultrasonic characteristics such as a resonance frequency. The ultrasonic transducer elements are switched in accordance with the ultrasonic focal length.
According to the apparatus which switches the transducer elements, assume that a given ultrasonic transducer element is switched to another one in order to change a focal point when an object is scanned with an ultrasonic beam from the given transducer element and an image of the object is displayed on a monitor. In this case, the position of the object displayed on the monitor is undesirably changed in correspondence with the position of the switched transducer element. For example, if transducer elements A and B are arranged to be separated by an angular interval of 180.degree., an object image obtained by transducer B is displayed at a position separated from an object image obtained by transducer element A by an angular interval of 180.degree..